Several experiments are proposed to elucidate the functional role of inhibitory GABAergic circuits in primary and secondary somatic sensory cortex of cats. This will be accomplished by recording individual neuronal respnoses to servo-controlled tactile stimuli before, during, and after microiontophoretic applicaton of drugs (e.g., bicculline) that selectively block post-synaptic GABA receptors. The first experiment will investigate how GABAergic activity influences the spatial organization of cortical receptive fields by measuring changes in the magnitude of neuronal responses to stimuli of different indentation amplitudes at different receptive field positions. This experiment will study drug induced changes in the summated response profiles. A second experiment will examine stimulus-response functions to vibratory stimuli and will determine whether cells with phasic responses to a sustained vibration show sustained periodically entrained discharges when GABA receptors are blocked and whether GABAergic transmission influences the range of frequencies of vibratory stimuli that can activate a cortical cell. A final experiment will examine the role of GABAergic circuits in controlling convergence and integration of inputs from different peripheral mechanoreceptors. A condition-test paradigm will be used to determine if a neuron's response to a preferred range of vibratory frequencies is altered by the simultaneous presence of a second, conditioning vibration that is applied near the cutaneous site of the test stimulus either within or near the receptive field of the cell. The conditioning vibrations are to be applied in the preferred or non-preferred range of frequencies and thereby activate the same or a different population of peripheral receptors then are associated with the optimal vibration frequency being applied through the test stimulus. Collectively, these experments will indicate possible contributions of GABAergic circuits to the interplay of afferent activity that determines a cortical cell's responsiveness and the range of peripheral receptors that influence its activity. These results should indicate, more generally, the role of GABAergic inhibitory circuits in the unique neuronal responses that characterize SI and SII.